Untreated cedar planks can be found at most specialty houseware stores, home improvement centres or grocery stores.
Salmon can also be grilled on a piece of heavy-duty foil instead of the cedar plank.
Plank can be soaked overnight before using as directed. While grilling, periodically check the plank, keeping a spray bottle handy. The plank should smoke but not burn. If plank starts to burn, just spray it with the water.